Conventionally, there have been proposed various types of seatbelt retractors which prevent a webbing from being drawn out in case of an emergency such as vehicle collision, and specifically seatbelt retractors which absorb energy acting on a vehicle occupant in case of an emergency such as vehicle collision and eventually, restrain the occupant securely by the webbing.
For instance, in a seatbelt retractor disclosed in Japanese Laid-open Patent Application Publication No. 2006-240618, a stopper mechanism fastens a ratchet wheel and a take-up drum in a relatively non-rotatable manner, and has a ring member screwed to engage on the outside through a threading engagement mechanism at a cylindrical portion of the ratchet wheel. The ratchet wheel is coupled to a spline shaft portion of a spindle shaft so as to be relatively non-rotatable. This ring member is fitted inside an engagement hole of the take-up drum and arranged in a manner relatively non-rotatable with regard to the take-up drum, and relatively movable in an axial direction of the take-up drum.
This threading engagement mechanism includes an internal thread portion formed on the inner periphery of the ring member, and an external thread portion formed on the outer periphery of the cylindrical portion of the ratchet wheel and being engaged with the internal thread portion. The ring member moves in the axial direction of the take-up drum when the ring member rotates relative to the cylindrical portion. Then, the axially outer end face of the ring member makes contact with the axially inner end face of a guiding portion of the ratchet wheel so that the ring member and the cylindrical portion are integrated and become relatively non-rotatable.